Magnetic field sensors can be realized in many forms. A variety of magnetic field sensors have been developed, many of them employing various wound-coil configurations. Other sensors rely on different physical principles. Thus, Hall-effect sensors, Magnetoresistive sensors and Magnetic wire sensors, as well as a version of an electron tube have been used. The present invention is concerned with wound-coil sensors.
A fluxgate sensor is used to detect and measure a magnetic field. Early fluxgate sensors consisted of a length of wire wound on a hollow tubular bobbin with a magnetically sensitive material placed in the centre of the bobbin, to form the core of the sensor. When the sensor is driven into saturation by alternate positive and negative pulses, the response of the sensor coil is a measure of the magnetic field strength parallel to the sensor's axis. Consequently, this type of sensor is referred to as a "parallel fluxgate sensor". Sensors can be configured to provide an "orthogonal fluxgate sensor".
Such sensors are used in a "magnetometer", which is a device that measures the strength of a magnetic field, and a "gradiometer" which is a device that measures the difference in magnetic field strength or gradient of the magnetic field.
There are several practical design considerations when using a sensor of the type just described. Principal considerations are sensitivity and balance. Balance is the most critical design consideration for a gradiometer, since several sensors are used to determine the strength of the magnetic field gradient. Sensitivity is also important, but it is much more easily controlled. If the sensors of a magnetic gradiometer are not exactly balanced, then the gradiometer can give a false output, i.e. it can indicate that there is a difference in the magnetic field when no such difference exists, or alternatively fail to detect if a magnetic difference does occur. Similarly, a device must be sensitive enough for its intended use.
In all known conventional sensors, a sensor has a coil wound about a bobbin and a core located in the bobbin. The core entirely fills the core of the bobbin and does not extend beyond the ends of the core. The core cannot be manipulated, and it is conventional to provide balance and compensation coils to achieve a balance between the sensor coils.